Substantial attention has been directed in recent years toward composite materials capable of exhibiting negative effective permeability and/or negative effective permittivity with respect to incident electromagnetic radiation. Such materials, often interchangeably termed artificial materials or metamaterials, generally comprise periodic arrays of electromagnetically resonant cells that are of substantially small dimension (e.g., 20% or less) compared to the wavelength of the incident radiation. Although the individual response of any particular cell to an incident wavefront can be quite complicated, the aggregate response the resonant cells can be described macroscopically, as if the composite material were a continuous material, except that the permeability term is replaced by an effective permeability and the permittivity term is replaced by an effective permittivity. However, unlike continuous materials, the resonant cells have structures that can be manipulated to vary their magnetic and electrical properties, such that different ranges of effective permeability and/or effective permittivity can be achieved across various useful radiation wavelengths.
Of particular appeal are so-called negative index materials, often interchangeably termed left-handed materials or negatively refractive materials, in which the effective permeability and effective permittivity are simultaneously negative for one or more wavelengths depending on the size, structure, and arrangement of the resonant cells. Potential industrial applicabilities for negative-index materials include so-called superlenses having the ability to image far below the diffraction limit to λ/6 and beyond, new designs for airborne radar, high resolution nuclear magnetic resonance (NMR) systems for medical imaging, microwave lenses, and other radiation processing devices.
One issue that arises in the realization of useful devices from such composite materials, including negative index materials, relates to device bandwidth. In particular, issues arise in relation to the spectral width of incident radiation for which negative effective permeability and/or negative effective permittivity is achieved. Accordingly, it would be desirable to spectrally broaden such composite materials with respect to their negative index behaviors, negative effective permeability behaviors, and/or negative effective permittivity behaviors. It would be further desirable to provide such spectral broadening while also providing a uniformity of response across a surface of the composite material. It would be still further desirable to provide for equalization and/or amplification of the response of such composite materials across the broadened spectrum of operation. Other issues arise as would be apparent to one skilled in the art in view of the present disclosure.